1. Field of the Invention
The present invention relates to an optical transmitter and an optical transmission system, and more particularly to an optical transmitter capable of producing an optical wideband modulated signal from a multi-channel video signal being a source signal, and an optical transmission system including such an optical transmitter.
2. Description of the Background Art
Various conventional systems have been proposed in the art for transmitting/distributing multi-channel video signals to subscriber premises. For example, an optical CATV system has been proposed in the art, including a down-converter for converting an IF signal (frequency: about 1 to 2.6 GHz) of satellite broadcasting such as BS digital, BS analog and CS digital to a signal of a lower frequency, i.e., lowering the frequency of a multi-channel video signal to 770 MHz or less. However, the number of programs increases as the broadcasting media are more digitalized, whereby there will be a shortage of the video signal band.
In view of this problem, new systems have been discussed, which are capable of transmitting high-frequency signals such as satellite broadcasting signals without frequency conversion and realizing high-frequency, wideband modulation while improving noise characteristics. See, for example, Japanese Laid-Open Patent Publication No. 2006-049977 (page 10, FIG. 1) and Japanese Laid-Open Patent Publication No. 2001-133824 (page 25, FIG. 1).
FIG. 11 is a block diagram showing a configuration of a conventional optical transmission system 100.
Referring to FIG. 11, the conventional optical transmission system 100 includes an optical transmitter 110 and an optical receiver 150 connected to each other via an optical fiber 170. The optical transmitter 110 includes an optical source 111, an optical branching section 112, an optical intensity modulation section 113, an optical phase modulation section 114, an optical combining section 115, an optical detecting section 116, and an optical transmitter section 118. The optical receiver 150 includes an optical receiver section 151 and an FM demodulation section 155. The optical intensity modulation section 113 is, for example, an optical SSB-SC (Single Side Band with Suppressed Carrier) modulator.
FIG. 12 is a schematic diagram showing exemplary signal spectra at different locations (12a to 12g) in the conventional optical transmission system 100.
First, the operation of the components of the optical transmitter 110 will be described.
The optical source 111 outputs an unmodulated optical signal having a frequency fa (wavelength λa) (hereinafter referred to as an “optical signal fa”). The optical branching section 112 splits the optical signal fa into two signals, which are inputted respectively to the optical intensity modulation section 113 and the optical phase modulation section 114. The optical intensity modulation section 113 receives an electric signal having a frequency fc (hereinafter referred to as an “electric signal fc”) (FIG. 12 (12a)). The optical phase modulation section 114 receives a first multi-channel signal including first to nth electric signals having frequencies f1 to fn (f1<fn, n is an integer), respectively, and a second multi-channel signal including oth to tth electric signals having frequencies fo to ft (fo<ft, o and t are integers) (FIG. 12 (12c)).
The optical intensity modulation section 113 subjects the input optical signal fa to an optical intensity modulation (or an optical amplitude modulation) based on the amplitude of the electric signal fc to output the resultant signal as an optical intensity-modulated signal (FIG. 12 (12b)). The optical phase modulation section 114 subjects the optical signal fa to an optical phase modulation (or an optical frequency modulation) based on the amplitude level of the first multi-channel signal and that of the second multi-channel signal to output the resultant signal as an optical phase-modulated signal (FIG. 12 (12d)).
The optical combining section 115 combines together the optical intensity-modulated signal outputted from the optical intensity modulation section 113 and the optical phase-modulated signal outputted from the optical phase modulation section 114 (FIG. 12 (12e)). The optical detecting section 116 may be a photodiode having squared detection characteristics, or the like, and performs an optical homodyne detection through a squared detection of the optical intensity-modulated signal and the optical phase-modulated signal combined together by the optical combining section 115 to thereby produce a wideband modulated signal, being the difference beat signal between the two optical signals. The wideband modulated signal is a phase-modulated signal obtained by down-converting the optical phase-modulated signal outputted from the optical phase modulation section 114, and the center frequency thereof is fc (FIG. 12 (12f)). The optical transmitter section 118 may be a semiconductor laser, or like, and performs a predetermined modulation, e.g., an optical intensity modulation, on the first to tth electric signals with the original signal being the wideband modulated signal outputted from the optical detecting section 116 to thereby transmit the resultant signal as an optical wideband modulated signal to the optical fiber 170.
The operation of the components of the optical receiver 150 will now be described. The optical receiver section 151 receives an optical wideband modulated signal transmitted through the optical fiber 170, and performs a photoelectric conversion to output a wideband modulated signal. The FM demodulation section 155 performs an FM demodulation on the wideband modulated signal to output a multi-channel signal in which the first multi-channel signal and the second multi-channel signal are mixed together (FIG. 12 (12g)).
However, when a high-frequency signal as represented by the second multi-channel signal in the conventional optical transmission system 100, or a signal of an even higher frequency (e.g., up to about 2.6 GHz), is inputted and transmitted, the bandwidth of the wideband modulated signal becomes too wide for the bandwidth of the conventional optical receiver section 151 and the conventional FM demodulation section 155. Thus, it is necessary to replace the components. The bandwidth B_FM of the wideband modulated signal produced by the optical transmitter 110 can be derived from the Komai-Carson law shown in Expression 1 below. In the expression, p is the peak factor representing the ratio between the maximum amplitude (the peak power) of the multi-channel signal and the average amplitude (the average power) thereof, ΔF is the frequency deviation [Hz/ch], N is the number of channels, and f_max is the highest frequency [Hz] of the multi-channel signal.B—FM=2×(p·ΔF·√N+f_max)   Exp. 1
Where the input multi-channel video signal has about 100 channels whose frequencies are up to about 2.6 GHz, and has a frequency deviation of 40 MHz/ch and a peak factor of 3.3, the wideband modulated signal outputted from the optical transmitter 110 will have a bandwidth of about 7.8 GHz. Therefore, it is necessary to increase the bandwidth of the optical receiver section 151 and that of the FM demodulation section 155. Moreover, there is a phenomenon that a component based on the wideband modulated signal is outputted from the FM demodulation section 155, which deteriorates the characteristics of the multi-channel signal component demodulated by the FM demodulation section 155. In order to avoid the component deterioration, it is necessary to set the center frequency of the wideband modulated signal to be very high, i.e., about 16.5 GHz. Therefore, it is necessary to, for example, replace components used for level adjustment such as amplifiers, in addition to the optical receiver section 151 and the FM demodulation section 155 used in the optical receiver 150.